Showing papers on "Capacitance published in 2014"

Reduced Mesoporous Co3O4 Nanowires as Efficient Water Oxidation Electrocatalysts and Supercapacitor Electrodes

Abstract: While electrochemical water splitting is one of the most promising methods to store light/electrical energy in chemical bonds, a key challenge remains in the realization of an efficient oxygen evolution reaction catalyst with large surface area, good electrical conductivity, high catalytic properties, and low fabrication cost. Here, a facile solution reduction method is demonstrated for mesoporous Co3O4 nanowires treated with NaBH4. The high-surface-area mesopore feature leads to efficient surface reduction in solution at room temperature, which allows for retention of the nanowire morphology and 1D charge transport behavior, while at the same time substantially increasing the oxygen vacancies on the nanowire surface. Compared to pristine Co3O4 nanowires, the reduced Co3O4 nanowires exhibit a much larger current of 13.1 mA cm-2 at 1.65 V vs reversible hydrogen electrode (RHE) and a much lower onset potential of 1.52 V vs RHE. Electrochemical supercapacitors based on the reduced Co3O4 nanowires also show a much improved capacitance of 978 F g-1 and reduced charge transfer resistance. Density-functional theory calculations reveal that the existence of oxygen vacancies leads to the formation of new gap states in which the electrons previously associated with the Co-O bonds tend to be delocalized, resulting in the much higher electrical conductivity and electrocatalytic activity.

Solid‐State Supercapacitor Based on Activated Carbon Cloths Exhibits Excellent Rate Capability

Abstract: Activated carbon cloth is used as an electrode, achieving an excellent areal capacitance of 88 mF/cm(2) (8.8 mF/g) without the use of any other capacitive materials. Significantly, when it is incorporated as part of a symmetric solid-state supercapacitor device, a remarkable charge/discharge rate capability is observed; 50% of the capacitance is retained when the charging rate increases from 10 to 10,000 mV/s.

Super Long-Life Supercapacitors Based on the Construction of Nanohoneycomb-Like Strongly Coupled CoMoO4–3D Graphene Hybrid Electrodes

Abstract: Nanohoneycomb-like strongly coupled CoMoO4 -3D graphene hybird electrodes are synthesized for supercapacitors which exhibit excellent specific capacitance and superior long-term cycle stability. The supercapacitor device can power a 5 mm-diameter LED efficiently for more than 3 min with a charging time of only 2 s, and shows high energy densities and good cycle stability.

Mobile terminal and method for controlling the same

Abstract: A force sensitive touch screen device can include a device body including a first region and a second region, a window on the device body and corresponding to the first and second regions, a display unit below the window corresponding to the first region, a key input unit on the second region, and a touch sensor including a first touch sensing layer, a second touch sensing layer, and a force sensing layer, in which the first and second touch sensing layers are between a polarizer layer and a thin film transistor (TFT) layer that can detect a position of a touch input by sensing a change of capacitance on the first sensing layer and the second sensing layer, the force sensing layer can detect a force of the touch input by sensing capacitance on the third sensing layer, and a finger scan sensor is installed in the key input unit.

Capacitance of carbon-based electrical double-layer capacitors

Abstract: Experimental electrical double-layer capacitances of porous carbon electrodes fall below ideal values, thus limiting the practical energy densities of carbon-based electrical double-layer capacitors. Here we investigate the origin of this behaviour by measuring the electrical double-layer capacitance in one to five-layer graphene. We find that the capacitances are suppressed near neutrality, and are anomalously enhanced for thicknesses below a few layers. We attribute the first effect to quantum capacitance effects near the point of zero charge, and the second to correlations between electrons in the graphene sheet and ions in the electrolyte. The large capacitance values imply gravimetric energy storage densities in the single-layer graphene limit that are comparable to those of batteries. We anticipate that these results shed light on developing new theoretical models in understanding the electrical double-layer capacitance of carbon electrodes, and on opening up new strategies for improving the energy density of carbon-based capacitors.

High-performance flexible asymmetric supercapacitors based on a new graphene foam/carbon nanotube hybrid film

Abstract: In this work, we report the fabrication of a new 3D graphene foam (GF)/carbon nanotube (CNT) hybrid film with high flexibility and robustness as the ideal support for deposition of large amounts of electrochemically active materials per unit area. To demonstrate the concept, we have deposited MnO2 and polypyrrole (Ppy) on the GF/CNT films and successfully fabricated lightweight and flexible asymmetric supercapacitors (ASCs). These ASCs assembled from GF/CNT/MnO2 and GF/CNT/Ppy hybrid films with high loading of electroactive materials in an aqueous electrolyte are able to function with an output voltage of 1.6 V, and deliver high energy/power density (22.8 W h kg−1 at 860 W kg−1 and 2.7 kW kg−1 at 6.2 W h kg−1). The rate performance can be further improved with less loading of electroactive materials (10.3 kW kg−1 at 10.9 W h kg−1). The ASCs demonstrate remarkable cycling stability (capacitance retention of 90.2–83.5% after 10 000 cycles), which is among the best reported for ASCs with both electrodes made of non-carbon electroactive materials. Also the ASCs are able to perfectly retain their electrochemical performance at different bending angles. These ASCs demonstrate great potential as power sources for flexible and lightweight electronic devices.

Metal–organic frameworks: a new promising class of materials for a high performance supercapacitor electrode

Abstract: A layered structure Ni-based MOF was synthesized and, for the first time, was used as the electrode material for a supercapacitor. It exhibited large specific capacitance, high rate capability and cycling stability. Capacitances of 1127 and 668 F g−1 can be achieved at rates of 0.5 and 10 A g−1, respectively. At the same time, over 90% performance was retained after 3000 cycles. These excellent electrochemical properties may be related to the intrinsic characteristics of Ni-based MOF materials.

Amorphous Ni(OH)2 @ three-dimensional Ni core–shell nanostructures for high capacitance pseudocapacitors and asymmetric supercapacitors

Abstract: A complex hydroxide/metal Ni(OH)2@Ni core–shell electrode was developed for a high-performance and flexible pseudocapacitor. Compared to the conventional Ni(OH)2 electrode, the as-prepared amorphous Ni(OH)2@ three-dimensional (3D) Ni core–shell electrode shows a large specific capacitance of 2868 F g−1 at a scan rate of 1 mV s−1 and a good cycling stability (3% degradation after 1000 cycles) at a scan rate of 100 mV s−1. Furthermore, the high rate capability with a specific capacitance of 2454 F g−1 can be achieved at a charge–discharge current density of 5 A g−1. An amorphous Ni(OH)2@3D Ni-AC based asymmetric supercapacitor could be cycled reversibly in the high-voltage region of 0–1.3 V, and the specific capacitance of 92.8 F g−1 at 1 A g−1. This research demonstrates that introduction of a metal core to conventional hydroxide supercapacitor electrodes could open up new opportunities for designing and developing high-performance supercapacitors.

Colossal pseudocapacitance in a high functionality–high surface area carbon anode doubles the energy of an asymmetric supercapacitor

Abstract: Here we demonstrate a facile template-free synthesis route to create macroscopically monolithic carbons that are both highly nitrogen rich (4.1–7.6 wt%) and highly microporous (SA up to 1405 m2 g−1, 88 vol% micropores). While such materials, which are derived from common chicken egg whites, are expected to be useful in a variety of applications, they are extremely promising for electrochemical capacitors based on aqueous electrolytes. The Highly Functionalized Activated Carbons (HFACs) demonstrate a specific capacitance of >550 F g−1 at 0.25 A g−1 and >350 F g−1 at 10 A g−1 in their optimized state. These are among the highest values reported in the literature for carbon-based electrodes, including for systems such as templated carbons and doped graphene. We show that HFACs serve as ideal negative electrodes in asymmetric supercapacitors, where historically the specific capacitance of the oxide-based positive electrode was mismatched with the much lower specific capacitance of the opposing AC. An asymmetric cell employing HFACs demonstrates a 2× higher specific energy and a 4× higher volumetric energy density as compared to the one employing a high surface area commercial AC. With 3.5 mg cm−2 of HFAC opposing 5.0 mg cm−2 of NiCo2O4/graphene, specific energies (active mass normalized) of 48 W h kg−1 at 230 W kg−1 and 28 W h kg−1 at 1900 W kg−1 are achieved. The asymmetric cell performance is among the best in the literature for hybrid aqueous systems, and actually rivals cells operating with a much wider voltage window in organic electrolytes.

Multifunctional g-C3N4 Nanofibers: A Template-Free Fabrication and Enhanced Optical, Electrochemical, and Photocatalyst Properties

Abstract: We have developed a facile, scale up, and efficient method for the preparation of graphitic-C3N4 nanofibers (GCNNFs) as electrodes for supercapacitors and photocatalysts. The as-synthesized GCNNFs have 1D structure with higher concentration of nitrogen that is favorable for higher conductivity and electrochemical performance. Secondly, the high surface area of GCNNF provides a large electrode–electrolyte contact area, sufficient light harvesting and mass transfer, as well as increased redox potential. Thus, the GCNNF supercapacitor electrode shows high capacitance of 263.75 F g–1 and excellent cyclic stability in 0.1 M Na2SO4 aqueous electrolyte with the capacitance retention of 93.6% after 2000 cycles at 1 A g–1 current density. GCNNFs exhibit high capacitance of 208 F g–1 even at 10 A g–1, with the appreciable capacitance retention of 89.5%, which proves its better rate capability. Moreover, the GCNNF shows enhanced photocatalytic activity in the photodegradation of RhB in comparison to the bulk graphit...

Facile Synthesis of MnO2/CNTs Composite for Supercapacitor Electrodes with Long Cycle Stability

Abstract: The MnO2/carbon nanotubes (CNTs) composites were prepared through a modified one-pot reaction process, in which CNTs were coated by cross-linked MnO2 flakes uniformly. The composition, morphology, and microstructure of the products were characterized using TG, XRD, XPS, Raman, FESEM, TEM, and STEM. It reveals that the MnO2 layer stands on the sidewalls of the inner nanotubes uniformly about 50 nm thick, and the loading of MnO2 on the CNTs reaches 84%. Furthermore, the supercapacitive performances were investigated by cyclic voltammogram (CV), galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS). The experimental results indicate that the composite exhibits not only high specific capacitance of 201 F g–1 and rate capability (the specific capacitance at 20 A g–1 is 70% of that at 1 A g–1), but also excellent cycle stability (no obvious capacitance decay after 10 000 cycles at 1 A g–1). An asymmetric electrochemical capacitor was assembled by using the obtained MnO2/CNTs composite...

3D micro-porous conducting carbon beehive by single step polymer carbonization for high performance supercapacitors: the magic of in situ porogen formation

Abstract: We report non-templated synthesis of interconnected microporous carbon (IMPC) sheets having beehive morphology by direct pyrolysis of poly(acrylamide-co-acrylic acid) potassium salt in inert atmosphere without any activation. The presence of the alkali metal in the selected polymer precursor results in a high specific surface area of 1327 m2 g−1. Importantly, 80% of the pore volume is contributed by micropores with pore size ranging from 1–2 nm which is ideal for use as an electrode for supercapacitors. Whereas the rest of the surface area was contributed by a small fraction of mesopores and macropores due to the interconnected structure. The presence of three different types of pores make the material ideal for supercapacitor electrodes. IMPC was tested as an electrode in both aqueous and non-aqueous supercapacitors. All the aqueous measurements were done in 1 M H2SO4 solution with a potential window 1 V. A specific capacitance of 258 F g−1 was realized at a constant charge–discharge current of 0.5 A g−1 and it maintained at a value of 150 F g−1 at 30 A g−1. A long cycle stability of 90% capacitance retention was observed after 5000 charge–discharge cycles at a current density of 2 A g−1. At the highest power density 13 600 W kg−1 the energy density was found to be 3.1 W h kg−1. Non aqueous performance was tested in the presence of 1 M LiPF6 in ethylene carbonate–di-methyl carbonate with 5 mg active material loading. A specific capacitance of 138 F g−1 was obtained at a current density of 0.25 A g−1 and it retained at a value of 100 F g−1 at 10 A g−1. The material can deliver an energy density of 31 W h kg−1 at its highest power density of 11 000 W kg−1 in a two electrode system based on active material loading.

Core-Double-Shell, Carbon Nanotube@Polypyrrole@MnO2 Sponge as Freestanding, Compressible Supercapacitor Electrode

Abstract: Design and fabrication of structurally optimized electrode materials are important for many energy applications such as supercapacitors and batteries. Here, we report a three-component, hierarchical, bulk electrode with tailored microstructure and electrochemical properties. Our supercapacitor electrode consists of a three-dimensional carbon nanotube (CNT) network (also called sponge) as a flexible and conductive skeleton, an intermediate polymer layer (polypyrrole, PPy) with good interface, and a metal oxide layer outside providing more surface area. These three components form a well-defined core-double-shell configuration that is distinct from simple core-shell or hybrid structures, and the synergistic effect leads to enhanced supercapacitor performance including high specific capacitance (even under severe compression) and excellent cycling stability. The mechanism study reveals that the shell sequence is a key factor; in our system, the CNT–PPy–MnO2 structure shows higher capacitance than the CNT–MnO...

Novel graphene/carbon nanotube composite fibers for efficient wire-shaped miniature energy devices

Abstract: Novel nanostructured composite fibers based on graphene and carbon nanotubes are developed with high tensile strength, electrical conductivity, and electrocatalytic activity. As two application demonstrations, these composite fibers are used to fabricate flexible, wire-shaped dye-sensitized solar cells and electrochemical supercapacitors, both with high performances, for example, a maximal energy conversion efficiency of 8.50% and a specific capacitance of ca. 31.50 F g(-1). These miniature wire-shaped devices are further shown to be promising for flexible and portable electronic facilities.

Flexible and Wire-Shaped Micro-Supercapacitor Based on Ni(OH)2-Nanowire and Ordered Mesoporous Carbon Electrodes

Abstract: Portable and multifunctional electronic devices are developing in the trend of being small, flexible, roll-up, and even wearable, which asks us to develop flexible and micro-sized energy conversion/storage devices. Here, the high performance of a flexible, wire-shaped, and solid-state micro-supercapacitor, which is prepared by twisting a Ni(OH)2-nanowire fiber-electrode and an ordered mesoporous carbon fiber-electrode together with a polymer electrolyte, is demonstrated. This micro-supercapacitor displays a high specific capacitance of 6.67 mF cm-1 (or 35.67 mF cm-2) and a high specific energy density of 0.01 mWh cm-2 (or 2.16 mWh cm-3), which are about 10-100 times higher than previous reports. Furthermore, its capacitance retention is 70% over 10 000 cycles, indicating perfect cyclic ability. Two wire-shaped micro-supercapacitors (0.6 mm in diameter, approximate to 3 cm in length) in series can successfully operate a red light-emitting-diode, indicating promising practical application. Furthermore, synchrotron radiation X-ray computed microtomography technology is employed to investigate inner structure of the micro-device, confirming its solid-state characteristic. This micro-supercapacitor may bring new design opportunities of device configuration for energy-storage devices in the future wearable electronic area.

Ultrahigh energy density realized by a single-layer β-Co(OH)2 all-solid-state asymmetric supercapacitor.

Abstract: A conceptually new all-solid-state asymmetric supercapacitor based on atomically thin sheets is presented which offers the opportunity to optimize supercapacitor properties on an atomic level. As a prototype, β-Co(OH)2 single layers with five-atoms layer thickness were synthesized through an oriented-attachment strategy. The increased density-of-states and 100 % exposed hydrogen atoms endow the β-Co(OH)2 single-layers-based electrode with a large capacitance of 2028 F g−1. The corresponding all-solid-state asymmetric supercapacitor achieves a high cell voltage of 1.8 V and an exceptional energy density of 98.9 Wh kg−1 at an ultrahigh power density of 17 981 W kg−1. Also, this integrated nanodevice exhibits excellent cyclability with 93.2 % capacitance retention after 10 000 cycles, holding great promise for constructing high-energy storage nanodevices.

Scalable self-growth of Ni@NiO core-shell electrode with ultrahigh capacitance and super-long cyclic stability for supercapacitors

Abstract: Three-dimensional (3D) electrodes have been demonstrated to be promising candidates for high-performance supercapacitors because of their unique architectures and outstanding electrochemical properties. However, the fabrication process for current 3D electrodes is not scalable. Herein, a novel and cost-effective activation process has been developed to macroscopically produce 3D porous Ni@NiO core-shell electrodes with enhanced electrochemical properties. The porous Ni@NiO core-shell electrode obtained by activated commercial Ni foam (NF) in a 3 M HCl solution yields an ultrahigh areal capacitance of 2.0 F cm−2 at a high current density of 8 mA cm−2, which is substantially higher than that of most reported 3D NF-based electrodes. Moreover, the activated NF (ANF) electrode exhibited super-long cycling stability. Owing to the increased accessible surface area and continual formation of electrochemically active NiO during cycling, the areal capacitance of the ANF electrode did not exhibit any decay and instead increased from 0.47 to 1.27 F cm−2 after 100 000 cycles at 100 mV s−1. This is the best cycling stability achieved by a 3D NF-based electrode. Additionally, a high-performance asymmetrical supercapacitor (ASC) device based on the as-prepared ANF cathode and a reduced graphene oxide (RGO) anode was also prepared. The ANF//RGO-ASC device was able to deliver a maximum energy density of 1.06 mWh cm−3 and a maximum power density of 0.42 W cm−3. Researchers from China have discovered a cost-effective way to produce supercapacitors on large scales using nickel foam. This three-dimensional porous metal is an ideal electrode for high-capacity energy storage because of its lightweight, corrosion-resistant structure. To achieve supercapacitance, however, researchers must insert active substances, such as graphene, deep into the nickel pores. Xihong Lu and colleagues from Sun Yat-Sen University solved this problem by immersing commercial-grade nickel foam into hot hydrochloric acid for several minutes. The one-step reaction pitted the formerly smooth nickel foam surface and created a thin outer ‘shell’ of nickel oxide that surrounded an inner nickel ‘core’. Electrochemical experiments revealed that the favorable core–shell structure, combined with a more accessible surface area achieved from the acid etching, yielded an energy-dense supercapacitor electrode that was effective for more than 100,000 charge–recharge cycles. A novel and cost-effective activation process has been developed to macroscopically produce three-dimensional (3D) porous Ni@NiO core-shell electrode by activated Ni foam (ANF) in HCl aqueous solution. The ANF electrode yielded a remarkable areal capacitance of 2.0 F cm−2 at a high current density of 8 mA cm−2 and exhibited ultrahigh long-term cycling stability without any decay of capacitance after 100 000 cycles.

Display device with touch sensor

Abstract: A common electrode for a display, which is originally provided in a liquid crystal display element, is also used as one (drive electrode) of a pair of electrodes for a touch sensor, and the other (detection-electrode-for-the-sensor) of the pair of electrodes is newly formed. An existing common drive signal as a drive signal for display is used in common for a drive signal for the touch sensor. A capacitance is formed between the common electrode and the detection-electrode-for-the-sensor, and touch detection is performed by utilizing a change of this capacitance caused by a finger touch of a user. Thus, a display device with a touch sensor is also applicable to a mobile device in which electric potential of the user is inconstant in many cases. The newly-provided electrode is only the detection-electrode-for-the-sensor, and it is unnecessary to newly prepare a drive signal for the touch sensor.

Direct Growth of NiCo2S4 Nanotube Arrays on Nickel Foam as High-Performance Binder-Free Electrodes for Supercapacitors

Abstract: Arrays of NiCo2 S4 nanotubes on nickel foam were prepared by means of a two-step method, and were directly applied as a binder-free supercapacitor electrode. Such a binder-free method enables intimate contact between the current collector and the active materials, and can effectively improve ion and charge transportation. As a result, the electrochemical performances of supercapacitors can be improved. The as-prepared NiCo2 S4 nanotubes/Ni foam electrode shows a high specific capacitance (738 F g-1 at 4 A g-1 ), excellent rate capability (78 % capacitance retention at 32 A g-1 ), and good cycling stability (retention capacity of 93.4 % after 4000 cycles), which suggests its promising application for electrochemical capacitors.

Defect density and dielectric constant in perovskite solar cells

Abstract: We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH3NH3PbIxCl1−x perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 1011/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

Shape-controlled synthesis of NiCo2S4 and their charge storage characteristics in supercapacitors

Abstract: In this work, a facile hydrothermal approach for the shape-controlled synthesis of NiCo2S4 architectures is reported. Four different morphologies, urchin-, tube-, flower-, and cubic-like NiCo2S4 microstructures, have been successfully synthesized by employing various solvents. The obtained precursors and products have been characterized by X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy. It is revealed that the supersaturation of nucleation and crystal growth is determined by the solvent polarity and solubility, which can precisely control the morphology of NiCo2S4 microstructures. The detailed electrochemical performances of the various NiCo2S4 microstructures are investigated by cyclic voltammetry and galvanostatic charge–discharge measurements. The results indicate that the tube-like NiCo2S4 exhibits promising capacitive properties with high capacitance and excellent retention. Its specific capacitance can reach 1048 F g−1 at the current density of 3.0 A g−1 and 75.9% of its initial capacitance is maintained at the current density of 10.0 A g−1 after 5000 charge–discharge cycles.

Highly Conductive Ordered Mesoporous Carbon Based Electrodes Decorated by 3D Graphene and 1D Silver Nanowire for Flexible Supercapacitor

Abstract: Ordered mesoporous carbon (OMC) is considered one of the most promising materials for electric double layer capacitors (EDLC) given its low-cost, high specific surface area, and easily accessed ordered pore channels. However, pristine OMC electrode suffers from poor electrical conductivity and mechanical flexibility, whose specific capacitance and cycling stability is unsatisfactory in flexible devices. In this work, OMC is coated on the surface of highly conductive three-dimensional graphene foam, serving as both charge collector and flexible substrate. Upon further decoration with silver nanowires (Ag NWs), the novel architecture of Ag NWs/3D-graphene foam/OMC (Ag-GF-OMC) exhibits exceptional electrical conductivity (up to 762 S cm−1) and mechanical robustness. The Ag-GF-OMC electrodes in flexible supercapacitors reach a specific capacitance as high as 213 F g−1, a value five-fold higher than that of the pristine OMC electrode. Moreover, these flexible electrodes also exhibit excellent long-term stability with >90% capacitance retention over 10 000 cycles, as well as high energy and power density (4.5 Wh kg−1 and 5040 W kg−1, respectively). This study provides a new procedure to enhance the device performance of OMC based supercapacitors, which is a promising candidate for the application of flexible energy storage devices.

Asymmetric supercapacitors based on nano-architectured nickel oxide/graphene foam and hierarchical porous nitrogen-doped carbon nanotubes with ultrahigh-rate performance

Abstract: A pulsed laser deposition process using ozone as an oxidant is developed to grow NiO nanoparticles on highly conductive three-dimensional (3D) graphene foam (GF). The excellent electrical conductivity and interconnected pore structure of the hybrid NiO/GF electrode facilitate fast electron and ion transportation. The NiO/GF electrode displays a high specific capacitance (1225 F g−1 at 2 A g−1) and a superb rate capability (68% capacity retention at 100 A g−1). A novel asymmetric supercapacitor with high power and energy densities is successfully fabricated using NiO/GF as the positive electrode and hierarchical porous nitrogen-doped carbon nanotubes (HPNCNTs) as the negative electrode in aqueous KOH solution. Because of the high individual capacitive performance of NiO/GF and HPNCNTs, as well as the synergistic effect between the two electrodes, the asymmetric capacitor exhibits an excellent energy storage performance. At a voltage range from 0.0 to 1.4 V, an energy density of 32 W h kg−1 is achieved at a power density of 700 W kg−1. Even at a 2.8 s charge–discharge rate (42 kW kg−1), an energy density as high as 17 W h kg−1 is retained. Additionally, the NiO/GF//HPNCNT asymmetric supercapacitor exhibits excellent cycling durability, with 94% specific capacitance retained after 2000 cycles.